A gas turbine engine generally includes at least one compressor to pressurize air to be channeled into a combustor. The engine may include at least one combustor in which at least a portion of the channeled pressurized air is mixed with fuel and ignited. Hot gasses from the compressor flow downstream through at least one turbine section. Each turbine section has rotating blades rotating about an axis and contained within an engine housing. The turbine section or sections may power any one of the compressor, a fan, a shaft, and/or may provide thrust through expansion through a nozzle, for example.
The turbine blades and/or stator vanes in the turbine portions must be able to withstand thermal stresses due to high temperatures and large temperature fluctuations as well as forces due to the high rotational speed experienced during normal operation of the turbine. As the pressure ratio and efficiency of turbines have increased, the thermal stresses the high pressure and low pressure turbine portion are exposed to have also increased. Accordingly, in combination with manufacturing components of the turbine (e.g. turbine blades and stator vanes) from a high-temperature resistant material, effective cooling of the turbine blades, stator vanes and other components have become increasingly important and challenging. To counteract the radiation and convection of heat to the turbine section, several heat removal techniques have been employed in the past; fluid cooling is generally employed to prolong the life of the turbine components. Further, small cooling holes have been drilled though the blade at angles optimized to remove heat and provide a thermal barrier on the surface of each airfoil surface of the turbine blades and stator vanes. Passages are also formed within the turbine and/or stator vanes to provide convection cooling of the surface of each airfoil.
The desire for increased cooling efficiency within turbine engine has led to complex internal cooling passages within turbine components. Conventional techniques for manufacturing engine parts and components involve the process of investment or lost-wax casting. One example of investment casting involves the manufacture of a typical blade used in a gas turbine engine. A turbine blade and/or stator vane typically includes hollow airfoils that have radial channels extending along the span of a blade having at least one or more inlets for receiving pressurized cooling air during operation of the engine. Various cooling passages in a blade typically include a serpentine channel disposed in the middle of the airfoil between the leading and trailing edges. The airfoil typically includes inlets extending through the blade for receiving pressurized cooling air, which include local features such as short turbulator ribs or pins for increasing the heat transfer between the heated sidewalls of the airfoil and the internal cooling air.
The manufacture of these turbine blades, typically from high strength, superalloy metal materials, involves numerous steps as shown in FIGS. 1-4. As shown in FIG. 1, forming a cast component using traditional investment casting typically includes steps of: machining of dies for the outer wax structure and for ceramic cores 101, molding and firing the ceramic cores 102, molding a wax pattern with ceramic core 103, wax assembly prep 104, dipping the wax assembly in ceramic slurry 105, drying the ceramic slurry to provide a shell 106, dewaxing the shell 107, casting and leaching 108, and drilling cooling holes 109.
In the abovementioned process, a precision ceramic core 200 is manufactured to conform to the serpentine cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. The ceramic core 200 is assembled inside two die halves which form a space or void therebetween that define the resulting metal portions of the blade. A relatively rigid wax and/or plastic is injected into the assembled dies to fill the void and surround the ceramic core 200, at which point the ceramic core 200 is encapsulated within the wax. The two die halves are split apart and removed to expose and remove the rigid wax and/or plastic that has the precise configuration of the desired blade formed of a molded wax 211. The molded wax blade 211 with encapsulated ceramic core 200 is then attached to a wax tree structure 212. The wax tree structure 212 is formed of a paraffin wax or any wax that is less rigid than the wax used to form the molded wax blade 211. Because the wax of the wax tree 212 will ultimately define a flowpath for molten metal into the ceramic mold, the dimensional accuracy of the outer surface of the wax used to form the tree structure 212 is less crucial. Thus, a softer wax is generally used to form the individual paths of the wax tree 212 than for the precisely molded wax blade 211 of the desired wax blade. The wax blade 211 requires pins 205 for holding the core in place. The tree structure 212 may include a funnel shaped portion 214 for adding molten metal to the mold. As shown in FIGS. 2-4, the tree structure 212 also includes a ceramic filter 213 for filtration of molten metal in the casting operation.
Ceramic filters generally known in the art include ceramic foam filters (CFF) like the ceramic filter 213 as shown in FIGS. 2-3. These filters are formed by impregnating reticulated polyurethane foam with ceramic slip, removing the excess slip by squeezing the foam, and then drying and firing the body forming a CFF. Other known ceramic filters include symmetric filters. More recently, ceramic filters have been made using various additive technologies. For example, U.S. Patent Application Pub. No. 2016/0038866 A1 entitled “ceramic filters” describes an additively manufactured ceramic filter. Another example is “Advanced Filtration to Improve Single Crystal Casting Yield—Mikro Systems,” available at the National Energy Technology Laboratory (NETL) web site. These filters are sold as stand-alone filters that may be incorporated in the wax tree 212 as shown in FIG. 2, and then incorporated into the ceramic mold as shown in FIG. 3.
After wax injection and the attachment of wax passageways 212 which form the wax tree structure, the entire wax tree structure 212, ceramic filter 213, and wax turbine blade 211 is then coated with a ceramic material to form a ceramic shell 206, 204 as shown in FIG. 3. Then, the wax is melted and removed from the ceramic shell 206, leaving a corresponding void or space 201, 207 between the ceramic shell 206 and the internal ceramic core 200. Further, once the wax tree structure 212 is melted, the ceramic shell 204 defines a flow path in fluid communication with the void or space 201, 207. After the wax is removed, the ceramic core is held in place by pins 205. As shown in FIG. 4, molten superalloy metal 208 is then poured into the shell 206 through the flow path defined by a portion of the ceramic shell 204. The molten superalloy may include any one of stainless steel, aluminum, titanium, INCONEL® 625, INCONEL® 718, INCONEL® 188, cobalt chrome, nickel, among other metal materials or any alloy; such as nickel (Ni) superalloys, and/or Ni superalloy single crystal alloys. For example, the above alloys may include materials with trade names, Haynes 188®, Haynes 625®, Super Alloy INCONEL® 625™, CHRONIN® 625, ALTEMP® 625, NICKELVAC® 625, NICROFER® 6020, INCONEL® 188, and any other material having material properties attractive for the formation of components using the above-mentioned techniques. The molten superalloy metal 208 fills the voids 201, 207 and encapsulates the ceramic core 200 contained in the shell 206. The molten metal 208 is cooled and solidifies, and then the external ceramic shell 206 and internal ceramic core 202 are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found. In order to provide a pathway for removing the ceramic core material via a leaching process, a ball chute (not shown) and the tip pins (e.g. reference 505 in FIG. 11) must be provided. Generally, after the leaching process, a ball chute and tip pin holes within the turbine blade must be subsequently brazed shut.
The cast turbine blade 208 typically undergoes additional post-casting modifications, such as drilling of suitable rows of film cooling holes through the sidewalls of the airfoil as desired for providing outlets for the internally channeled cooling air which then forms a protective cooling air film or blanket (generally referred to as film cooling) over the external surface of the airfoil during operation in the gas turbine engine. After the turbine blade is removed from the ceramic mold, pins 205 which held the ceramic core 200 form a passageway that is later brazed shut to provide the desired pathway of air through the internal voids of the cast turbine blade. However, these post-casting modifications are limited and given the ever increasing complexity of turbine engines and the recognized efficiency improvements provided by certain cooling circuits inside turbine blades, more complicated and intricate internal geometries are required. While investment casting is capable of manufacturing these parts, positional precision and intricate internal geometries become more complex to manufacture using these conventional manufacturing processes and thus increase manufacturing time and expense significantly. Accordingly, it was desirable to provide an improved casting method for three dimensional components having intricate internal voids and cooling circuits.
Additive manufacturing techniques, and 3-D printing allowed molds to be manufactured without the toolpath and/or molding limitations associated with subtractive manufacturing. For example, methods for using 3-D printing to produce a ceramic core-shell mold are described in U.S. Pat. No. 8,851,151 assigned to Rolls-Royce Corporation. The methods for making the molds include powder bed ceramic processes such as disclosed U.S. Pat. No. 5,387,380 assigned to Massachusetts Institute of Technology, and selective laser activation (SLA) such as disclosed in U.S. Pat. No. 5,256,340 assigned to 3D Systems, Inc. The ceramic core-shell molds according to the '151 patent are significantly limited by the printing resolution capabilities of these processes. As shown in FIG. 5, core portion 301 and shell portion 302 of integrated core-shell mold 300 is held together via a series of tie structures 303 provided at the bottom edge of the mold 300. Cooling passages are proposed in the '151 patent that include staggered vertical cavities joined by short cylinders, the length of which is nearly the same as its diameter. A superalloy turbine blade is then formed in the core-shell mold using known techniques disclosed in the '151 patent, and incorporated herein by reference. After a turbine blade is cast in one of these core-shell molds, the mold is removed to reveal a cast superalloy turbine blade.
There still remains the need to prepare ceramic core-shell molds produced using higher resolution methods that are capable of providing fine detail cast features in the end-product of the casting process such as fine resolution capability necessary to print filaments extending between the core and shell portion of the mold of sufficiently small size and quantity to result in effusion cooling holes in the finished turbine blade or stator vane, for example. In the case of earlier powder bed processes, such as disclosed in U.S. Pat. No. 5,387,380 assigned to Massachusetts Institute of Technology, the action of the powder bed recoater arm precludes formation of sufficiently fine filaments extending between the core and shell to provide an effusion cooling hole pattern in the cast part. Other known techniques such as selective laser activation (SLA) such as disclosed in U.S. Pat. No. 5,256,340 assigned to 3D Systems, Inc. that employ a top-down irradiation technique may be utilized in producing an integrated core-shell mold in accordance with the present invention. However, the available printing resolution of these systems significantly limits the ability to make filaments of sufficiently small size to serve as effective cooling holes in the cast final product.
While the above-mentioned processes can be used to form integrated core-shell mold, it is advantageous to manufacture a core-shell mold using direct light processing (DLP). DLP differs from the above discussed powder bed and SLA processes in that the light curing of the polymer occurs through a window at the bottom of a resin tank that projects light upon a build platform that is raised as the process is conducted. With DLP an entire layer of cured polymer is produced simultaneously, and the need to scan a pattern using a laser is eliminated. Further, the polymerization occurs between the underlying window and the last cured layer of the object being built. The underlying window provides support allowing thin filaments of material to be produced without the need for a separate support structure. In other words, producing a thin filament of material bridging two portions of the build object is difficult and was typically avoided in the prior art. For example, the '151 patent discussed above in the background section of this application used vertical plate structures connected with short cylinders, the length of which was on the order of their diameter. Staggered vertical cavities are necessitated by the fact that the powder bed and SLA techniques disclosed in the '151 patent require vertically supported ceramic structures and the techniques are incapable of reliably producing filaments. For example, round cooling holes generally have a diameter of less than 2 mm corresponding to a cooling hole area below 3.2 mm2. Production of a hole of such dimensions requires a resolution far below the size of the actual hole given the need to produce the hole from several voxels. This resolution is simply not available in a powder bed process. Similarly, stereolithography is limited in its ability to produce such filaments due to lack of support and resolution problems associated with laser scattering. But the fact that DLP exposes the entire length of the filament and supports it between the window and the build plate enables producing sufficiently thin filaments spanning the entire length between the core and shell to form a ceramic object having the desired cooling hole pattern. Although powder bed and SLA may be used to produce filaments, their ability to produce sufficiently fine filaments as discussed above is limited.
Further, in employing the above-mentioned DLP method of manufacturing a core-shell of the embodiment described above, various difficulties arise in integrating the use of a core-shell mold into an efficient manufacturing process. For example, the time required to form a core-shell mold having sufficient dimensional stability (e.g., wall thickness) using a DLP process may delay the manufacturing process and require the use of excess material. Further, in the molding process it may be desired to efficiently produce portions of a mold that do not require the same dimensional accuracy as is required in portions of the core-shell mold itself. For example, it may be desirable to produce passages for directing the flow of molten superalloy into a single or plurality of core-shell molds. Further, when forming a core-shell mold using a DLP process it may be desirable to improve the ease of removing the core-shell once the casting is completed. For example, the knockout process may be improved by producing a thinner core-shell, to reduce the likeliness that the cast product is damaged upon removal of the core-shell. It may also be desirable to control the thermal conductivity of the core-shell mold to control crystal growth and/or tailor the material properties of the cast component.